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Cross with lichen on stone, at Hermitage St Helier, Jersey

Lichen-covered tree, Isles of Scilly, UK. Grey, leafy lichen upper half of trunk; yellowy-green lichen on middle, lower half and up the extreme right side; and a shrubby lichen

A foliate (leafy) lichen on a branch with dark green mosses

Lichens are a symbiosis of at least two quite different organisms. The partnership always involves a fungus, which lives with one or more partners which can do photosynthesis. The photobiont partner may be a green alga and/or a cyanobacterium.^p5,6,13

The algae or bacteria live inside the fungus, and exchange nutrients with it. The lichen is different in shape and mode of life from any of the partners. It is a distinct form of life. Botanists did not know this until about 1875.